In a case for example of generating an internal gear, it is difficult to use a general hobbing machine and thus a gear shaper is usually used. For the generation of an internal gear by use of this gear shaper, gear shaping is usually performed in which an annular workpiece is cut into a given tooth profile by: linearly moving a pinion cutter in the axial direction of the workpiece while rotating the workpiece and the pinion cutter relative to each other; performing gear cutting on an area of the inner peripheral surface of the workpiece in the circumferential direction along the axial direction of the workpiece; and repeating this step for all the areas of the inner peripheral surface of the workpiece in the circumferential direction.
Here, internal gear machining with a hobbing machine is possible by using a special hobbing machine for internal gear cutting and a dedicated hobbing cutter. In a hobbing machine, a tool arbor is oriented substantially perpendicularly to the axial direction of the workpiece. For this reason, if a small-diameter internal gear is to be generated, the shaft of the tool and the workpiece will interfere with each other, which limits the dimensions of machinable workpieces. Thus, internal gear machining with a bobbing machine is limited to machining of relatively-large internal gears. In contrast, in skiving, the axis-crossing angle between the axis of the tool and the axis of the workpiece is about 30° or less. With such a positional relationship, the shaft of the tool and the workpiece are unlikely to interfere with each other. Thus, an internal gear can be machined regardless of whether the diameter of the workpiece is large or small.
Meanwhile, in gear shaping as mentioned above, in order to machine all the areas of a workpiece in the circumferential direction, a pinion cutter is moved back and forth (machining stroke and return stroke) in the axial direction of the workpiece a plurality of times corresponding to the circumferential length of the workpiece. Hence, an action that does not directly contribute to the gear cutting (return stroke) must be performed, which lowers the machining efficiency. In view of this, in recent years, skiving has drawn attention which requires no backward moving action of a pinion cutter or the like (return stroke) and involves cutting all the areas of the inner peripheral surface of an annular workpiece into a given axial tooth profile in one forward moving action of the pinion cutter or the like (machining stroke) in the axial direction of the workpiece with the workpiece and the pinion cutter rotated relative to each other.